In recent years, a resolution capacity of an image detecting apparatus in which, an image pickup optical system such as a microscope, a camera, and an endoscope is used has improved. Particularly, in a field of microscopes and optical recording, an optical system with almost no aberration has been realized, and a resolution capacity as an image pickup optical system has been restricted mainly by a diffraction limit of visible light. Whereas, as it has been disclosed in the following non-patent literatures, an optical material which takes a negative value of a refractive index (hereinafter, ‘negative refraction material’) has been realized. A proposal mentioning that an imaging of an ultra-high resolution beyond the diffraction limit (hereinafter, ‘perfect imaging’) is possible when a negative refraction material is used, has been made.
As it has been disclosed in non-patent literature 3, in a case other than a case in which, the refractive index takes a negative value, when a real part of a permittivity or a magnetic permeability is a negative value, a negative refraction phenomenon is observed for specifically polarized electromagnetic waves. Moreover, as it has been disclosed in non-patent literature 5, in a periodic structure such as a photonic crystal, as a result of a photonic band being reflected in a reciprocal lattice space, in spite of each of the refracting index, the permittivity, and the permeability being a positive material, the negative refraction phenomenon is observed for electromagnetic waves of specific wavelength which are polarized in a specific manner.
In view of the abovementioned circumstances, in this specification, a material which shows a negative refraction response for the specific electromagnetic waves will be called as a ‘material exhibiting negative refraction’. It is needless to mention that, an expression ‘a material exhibiting negative refraction’ is a concept of a wider sense than the negative refraction material.
As a material exhibiting negative refraction, apart from the photonic crystal mentioned above, materials such as metallic films, chiral substances, photonic crystals, meta-materials, left-handed materials, backward wave materials, and negative phase velocity media have been known.
According to non-patent literature 1, a material for which, both the permittivity and the permeability take the negative value, the refractive index also becomes a negative value. Furthermore, it has been shown that, such material satisfies a so-called extension of Snell's law, as it will be described later.
FIG. 16 shows refraction of light in an ordinary optical material (hereinafter called as ‘an ordinary optical material’) having a positive refractive index. When light is transferred from a medium 1 to a medium 2, the light is refracted at a boundary surface of both the media. At this time, Snell's law shown by the following expression (1) is satisfied.n1 sin θi=n2 sin θr  (1)
Here, θi denotes an angle of incidence, θr denotes an angle of refraction, n1 denotes a refractive index of the medium 1, and n2 denotes a refractive index of the medium 2.
Whereas, FIG. 17 shows refraction of light when the refractive index n2 of the medium 2 takes a negative value. As shown in FIG. 17, the light which is incident is refracted in a direction opposite to a direction of refraction shown in FIG. 16 with respect to a normal of the boundary surface. At this time, when the angle of refraction θr is let to be a negative value, the abovementioned Snell's law is satisfied.
FIG. 18 shows an imaging relationship by a convex lens 13 in which, an ordinary optical material is used. Light from an object point 11A on an object plane 11 is collected to an image point 12A on an image plane 12 by the convex lens 13. When the refractive index of the lens is positive, for imaging (collecting), a lens surface is required to have a finite curvature.
Whereas, a flat plate made of a material exhibiting negative refraction (hereinafter, appropriately called as a ‘negative refraction lens’) can collect light in spite of having an infinite curvature. FIG. 19 shows an imaging relationship by a negative refraction lens 14. Light from an object point 11B on an object plane 11 is collected at an image point 12B on an image plane 12 by the negative refraction lens 14.
In non-patent literature 11, a method for realizing a non-equal magnification imaging by forming a curved surface lens by a material exhibiting negative refraction has been disclosed. However, a condition for the perfect imaging being extremely strict, and since a material having a predetermined refractive index gradient in addition to exhibiting negative refraction is necessary, it is not realistic. Actually, all the negative refraction lenses realized in the world have almost a uniform refractive index spatially, and a surface thereof through which, light (electromagnetic waves) passes is a plane surface. Therefore, a spatially uniform flat plate made of a material exhibiting negative refraction will from here onward be called appropriately as a ‘negative refraction lens’.
Here, ‘spatially uniform’ means that, it is uniform with a scale which is larger than a wavelength of the electromagnetic waves. Consequently, it means that, in a case of realizing the negative refraction by an artificial structural material such as a photonic crystal and a meta-material, an effective refractive index (or an effective permittivity or an effecting permeability) which is attributable to the structure is spatially uniform.
In an imaging optical system such as a microscope, a theoretical upper limit value of resolution is determined by a diffraction limit. As it has been described in textbook of optics (non-patent literature 2 for example), according to Rayleigh criterion, the minimum distance between two resolvable points in about λ/NA. Here, λ is a wavelength used, and NA is a numerical aperture. Moreover, a structure smaller than the diffraction limit cannot carry out image dissection by an optical system.
Moreover, a microscope and an optical pickup which improve resolution by using an objective lens of a liquid immersion, an oil immersion, and a solid immersion have been proposed. These increase an effective NA (numerical aperture). Accordingly, the value of λ/NA corresponding to the diffraction limit is made smaller. Here, it is not possible to make the numerical aperture NA larger than a refractive index of a medium in which, an object plane is disposed. Therefore, an upper limit of the numerical aperture NA is about 1.5 to 2.0.
Light which has been emitted from the object point 11A on the object plane 11 is formed of two light waves namely, propagating light which reaches a far distance, and evanescent waves which are attenuated at a distance of about wavelength from the object point 11A. The propagating light corresponds to a low-frequency component of information on the image plane 11. Whereas, the evanescent waves correspond to a high-frequency component of the information on the image plane 11.
A boundary of the propagating light and the evanescent waves corresponds to a spatial frequency equivalent to 1/λ. Particularly, the evanescent waves have frequency within the object plane larger than 1/λ. Therefore, the evanescent waves have a wave-number component in a direction of propagation of light waves perpendicular to the evanescent waves becoming an imaginary number. Therefore, there is a rapid attenuation with receding from the object plane 11.
All the components of the propagating light on the other hand, do not advance to the optical system. A part of the propagating light is vignetted by an aperture in the optical system. Therefore, only a component smaller than NA/λ of the spatial frequency on the object plane 11 reaches the image plane 12. Eventually, in the information reaching the image point 12A, the high-frequency component from the information held by the object point 11A is missing. Accordingly, this becomes a spreading of a point image by diffraction, and restricts the resolution.
In non-patent literature 3 disclosed in recent years, it has been disclosed that the abovementioned evanescent waves are amplified in the negative refraction material. Therefore, in imaging by the negative refraction lens 14 shown in FIG. 19, the amplitude of the evanescent waves on the image plane 12 is shown to have been recovered to a level same as on the object plane 11. In other words, in the optical system shown in FIG. 19, both the propagating light and the evanescent waves are propagated from the object plane 11 to the image plane 12. Therefore, information of the object point 11B is perfectly reproduced at the imaging point 12B. This means that, when an imaging optical system in which, the negative refraction lens 14 is used, perfect imaging in which, the diffraction limit is not restricted is possible.
The abovementioned perfect imaging is not only a theoretical phenomenon. A negative refraction lens has actually been manufactured, and results of experiments have been reported. For instance, in non-patent literature 4, a meta-material in which, a rod and a metallic coil smaller than the wavelength, are arranged periodically, has been manufactured. Moreover, such meta-material has been reported to function as a negative refraction lens in a microwave range.
Moreover, in non-patent literature 5, a method of manufacturing a negative refraction material by using photonic crystal has been disclosed. In the photonic crystal in which, an air rod is disposed in a hexagonal lattice form in a dielectric substance, there exists a photonic band in which, the effective refractive index is isotropic as well as negative. Furthermore, the photonic crystal can be deemed as a two-dimensional uniform negative refraction material with respect to electromagnetic waves of a frequency band suitable for a photonic band.
There has been a theoretic counter argument as described in non-patent literature 6 for example, to the perfect imaging by the negative refraction lens, which lead to a controversy. However, in recent years, a theory of the negative refraction lens disclosed in non-patent literature 3 has been generally accepted.
In an optical system in which, an ordinary optical material is used, it is possible to create an aplanatic point, or in other words, a point at which, a spherical aberration and a coma aberration become zero simultaneously. An image by this optical system always becomes a virtual image. Here, when a negative refraction material is used, it is possible to form a real image by arranging an object plane at the aplanatic point (refer to non-patent literature 7 for example). In this manner, by using a negative refraction material, unique optical designing which was not available so far becomes possible.
Moreover, it has been known that for many metals, a real part of permittivity with respect to visible light becomes negative. For instance, according to non-patent literature 9, silver exhibits a negative permittivity for light of a wavelength in a range of 330 nm to 900 nm. Furthermore, according to non-patent literature 10, even in a chiral substance having a helical structure, there exists a photonic band exhibiting negative refraction.
Phenomenon of negative refraction has unique characteristics which are different from an ordinary optical material, such as having a negative angle of refraction, having a phase velocity and a group velocity in opposite directions, and an electric field, a magnetic field, and a wave number vector forming a left-handed system in this order.
A name of a material exhibiting negative refraction has not yet been established in general. Therefore, prefixing the abovementioned characteristics, a material exhibiting negative refraction is also called as a negative phase velocity material, a left-handed material, a backward-wave material, and a negative refraction material. In this specification, it will be treated as a type of such material exhibiting negative refraction. Such treatment does not contradict at all to a definition of the abovementioned material exhibiting negative refraction.
Moreover, there exist many names prefixed by a phenomenon of overlapping with names in which, a material or a structure is prefixed. For instance, a meta-material which is made of a metal resonator array is sometimes also called as a left-handed substance or a left-handed meta-material. Such materials are also to be included in materials exhibiting negative refraction.
In this manner, when a negative refraction lens which is formed of a negative refraction material is used, there is a possibility of realizing an imaging optical system of ultra-high resolution (perfect imaging) not constrained to diffraction limit (refer to non-patent literature 3, for example). Furthermore, even in a case of imaging only the propagating light, a unique optical design is possible (refer to non-patent literature 7, for example).    Non-patent literature 1: V. C. Veselago et al., Sov. Phys. Usp. 10, 509 (1968)    Non-patent literature 2: E. Hecht, “Optics”, 4th ed. (Addison-Wesley, Reading, Mass., 2002)    Non-patent literature 3: J. B. Pendry, Phys. Rev. Lett. 85, 3966 (2000)    Non-patent literature 4: D. R. Smith et al., Phys. Rev. Lett. 84, 4184 (2000)    Non-patent literature 5: M. Notomi, Phys. Rev. B62, 10696 (2000)    Non-patent literature 6: P. M. Valanju et al., Phys. Rev. Lett. 88, 187401 (2002)    Non-patent literature 7: D. Schurig et al., Phys. Rev. E70, 065601 (2004)    Non-patent literature 8: D. R. Smith et al., Appl. Phys. Lett. 82, 1506 (2003)    Non-patent literature 9: “Latest Optical Technology Handbook” by Tsujiuchi Junpei et al., (Published by Asakura Shoten)    Non-patent literature 10: J. B. Pendry, Science 306, 1353 (2004)    Non-patent literature 11: S. A. Ramakrishna et al., Phys. Rev. B69, 115115 (2004)
As it has been mentioned above, the negative refraction lens forms an image in which, the high-frequency component is retained by transmitting the evanescent waves. However, for generating arbitrarily some sort of optical image having the high-frequency component by using a negative refraction lens, or for detecting the high-frequency component from an optical image generated from an object etc. by a negative refraction lens, there are following issues related to a method of lighting and a method of detection.
Firstly, let us consider a case in which, an attempt is made to detect a desired high-frequency component by an optical image generated from an object etc. by a negative refraction lens. The perfect imaging of the negative refraction lens is always an equal (same size) magnification imaging. Moreover, even when the uniformly magnified image is supposedly magnified by an ordinary magnifying optical system, the evanescent waves are not transmitted to the magnified image. Consequently, the high-frequency component is lost, and it is not possible to detect the high-frequency component from the magnified image.
In other words, for detecting information of a certain desired high-frequency component, it is necessary that a detector is placed directly on an equal magnified-image plane by the negative refraction lens, and that the detector has a detection band spatial resolution larger than the desired high-frequency component.
Such type of problem exists similarly when an attempt is made to generate arbitrarily on an object, some sort of an optical image having the desired high-frequency component by using the negative refraction lens. In other words, it is necessary that illuminating light is modulated spatially on the equal magnified imaging plane (conjugate plane of object which is intended) by the negative refraction lens, and that a light source (an illuminating light source) has a modulation band (spatial resolution) larger than the desired high-frequency component.
Concrete problems in the detector and the light source in a case of detecting the high-frequency component of an optical image by the negative refraction lens will be described below, taking a microscope as an example. A two-point resolution of a microscope having an ordinary water-immersion objective lens is approximately 0.3 μm. The wavelength is let to be 0.5 μm, the numerical aperture is let to be 0.75, and refractive index of water is let to be 1.333.
Whereas, for a microscope with a negative refraction lens as the objective lens, to have ten times two-point resolution of the ordinary microscope, or in other words, to have two-point resolution of 0.03 μm, the detector or the light source is required to have resolution higher than that. This means that, in a case of using a two-dimensional image pickup element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor), a pixel interval (pixel dimension) has to be half of 0.03 μm, or in other words, not more than 0.015 μm.
Moreover, even in a case of a scanning microscope which detects a signal of an image by scanning by moving one or a plurality of detectors or light sources relatively with an object, a size of the detector and the light source has to be not more than 0.015 μm similarly as in a case of the image sensor described above.
However, manufacturing of such extremely small detector and light source is not easy. For instance, the smallest pixel interval in a CCD which is currently being used practically is approximately 2 μm. Consequently, for a CCD to achieve the resolution shown in the abovementioned example, or in other words, to achieve the pixel interval not more than 0.015 μm, it is necessary to have high densification of not less 130 times now onward. This technological degree of difficulty is extremely high.
Moreover, an SNOM (Scanning Near-Field Optical Microscope) is available as an optical microscope of super resolution which is currently being used practically. Even for an aperture portion of a front end of a probe which is used as a detector and a light source in the SNOM, a diameter is approximately 0.05 μm to 0.1 μm. This is three times larger than the condition, that is, the diameter of the detector and the light source is not more than 0.015 μm, shown in the abovementioned example.
The present invention is made in view of the abovementioned issues, and an object of the present invention is to provide a light system, a method of lighting which having a spatial resolution appropriate for a high-frequency component by the evanescent waves in a negative refraction lens, and a scanning optical microscope.